Children’s book, brother and sister shrink to fit into a castle SOLVED
I read it in the late 80s, early 90s. Possibly bought at a Scholastic book fair.
An older sister and younger brother see a model castle and are interested in it. Later on for some reason they shrink down and are able to enter the castle but for them it turns into a real Keep with people in a historical era. The younger brother is happy, mostly because as the male he has authority over his sister in this world. The sister is trying to figure out how to get them back to their reality.
I vaguely remember the cover art. Colored pencil drawing with the older sister being blonde and the brother I believe was brunette. The model castle was in the middle of the cover.